1. The Field of the Invention
The present invention relates generally to x-ray tubes that use a rotating anode target supported by a bearing assembly. More particularly, embodiments of the present invention relate to systems and devices concerned with improving the rate of heat transfer from the x-ray tube bearing assembly and related components so as to facilitate a relative increase in the life of the bearing assembly, and thus the x-ray device as a whole.
2. The Relevant Technology
X-ray producing devices are valuable tools that are used in a wide variety of industrial, medical, and other applications. For example, such equipment is commonly used in areas such as diagnostic and therapeutic radiology, semiconductor manufacture and fabrication, and materials analysis and testing. While they are used in various applications, the different x-ray devices share the same underlying operational principles. In general, x-rays, or x-ray radiation, are produced when electrons are produced, accelerated, and then impinged upon a material of a particular composition.
Typically, these processes are carried out within a vacuum enclosure. Disposed within the vacuum enclosure is an electron source, or cathode, and an anode, which is spaced apart from the cathode. In operation, electrical power is applied to a filament portion of the cathode, which causes a stream of electrons to be emitted by the process of thermionic emission. A high voltage potential applied across the anode and the cathode causes the electrons emitted from the cathode to rapidly accelerate towards a target surface, or focal track, positioned on the anode.
The accelerating electrons in the stream strike the target surface, typically a refractory metal having a high atomic number, at a high velocity and a portion of the kinetic energy of the striking electron stream is converted to electromagnetic waves of very high frequency, or x-rays. The resulting x-rays emanate from the target surface, and are then collimated through a window formed in the x-ray tube for penetration into an object, such as the body of a patient. As is well known, the x-rays can be used for therapeutic treatment, or for x-ray medical diagnostic examination or material analysis procedures.
In addition to stimulating the production of x-rays, the kinetic energy of the striking electron stream also causes a significant amount of heat to be produced in the anode. As a result, the anode typically experiences extremely high operating temperatures. However, the anode is not the only element of the x-ray tube subjected to such extreme operating temperatures.
In particular, a percentage of the electrons that strike the target surface do not generate x-rays, and instead simply rebound from the surface and then impact another xe2x80x9cnon-targetxe2x80x9d surfaces and structures within the x-ray tube evacuated enclosure. These are often referred to as xe2x80x9csecondaryxe2x80x9d electrons. These secondary electrons retain a large percentage of their kinetic energy after rebounding, and when they impact non-target surfaces, a significant amount of heat is generated that is conducted to various other elements, such as the bearing assembly, of the x-ray device.
The heat produced by secondary electrons, in conjunction with the high temperatures present at the target anode, often reaches levels high enough to damage portions of the x-ray tube structure. In particular, such extreme temperature operating conditions can shorten the operational life of the x-ray device, affect its efficiency and performance, and/or render it inoperable. Such high heat levels present special problems in the context of rotating anode type x-ray tubes.
In a typical rotating anode type x-ray tube, the anode is mounted to a shaft that is rotatably supported by a bearing assembly contained in a bearing housing. A stator serves to rotate the shaft, and the anode accordingly rotates as well. As the anode rotates, each point on the focal track is rotated into and out of the path of the electron beam generated by the cathode. In this way, the electron beam is in contact with a given point on the focal track for only short periods of time, thereby allowing the remaining portion of the focal track to cool during the time that it takes such given portion to rotate back into the path of the electron beam.
The rotating anode x-ray tube of this sort is used in a variety of applications, some of which require that the anode be rotated at relatively high speeds so as to maintain an acceptable heat distribution along the focal track. For instance, x-ray tubes used in mammography equipment have typically been operated with anode rotation speeds around 3500 revolutions per minute (rpm). However, the demands of the industry have continued to change and high-speed machines for mammography and other applications are now being produced that operate at anode rotation speeds of around 10,000 rpm and higher.
High rotational speeds coupled with extreme operating temperatures place tremendous stress and strain on the bearing assembly and related components of the rotating anode x-ray tube, resulting in a variety of undesirable consequences. For example, high rotational speeds and operating temperatures may cause increased vibration and noise in the bearing assembly. This increase in noise and vibration is undesirable, because it can be unsettling to a patient, particularly in applications such as mammography where the patient is in intimate contact with the x-ray machine. Moreover, noise and vibration can be distracting to the x-ray machine operator. Also, unchecked vibration can shorten the operating life of the x-ray tube. Finally, the quality of the images produced by the x-ray device are at least partly a function of the stability of the focal spot on the target surface. Thus, vibration may compromise the quality of the x-ray image by causing undesirable movement of the focal spot.
There are various mechanisms by which high rotational speeds and extreme operating temperatures may cause increased vibration and noise in the bearing assembly. For example, excessively high temperatures can melt the thin film metal lubricant that is typically present on the ball bearings of the bearing assembly. When the bearings cool, the metal lubricant may clump and then create rough spots in the bearing races. Upon subsequent start-up of the x-ray device, the balls travel at high speeds over the rough spots in the races, thereby creating vibration and noise. Moreover, repeated exposure to high temperatures can degrade the bearings, thereby reducing their useful life as well as that of the x-ray tube.
Another mechanism by which high rotational speeds and extreme operating temperatures generate vibration and noise relates to the physical arrangement of the components in the bearing assembly and bearing housing, and the materials from which those components are constructed. In particular, in some known designs, heat generated at the anode and as a result of secondary electron impacts is conducted directly to the bearing assembly by way of solid metal parts that collectively form a heat path between the anode and the bearing assembly. Thus, operational heat is readily transmitted from the anode to the bearing assembly and related components. Additional heat is also generated in the bearing assembly as a result of bearing friction, which increases as operating speeds increase.
As a result of physical arrangements such as that just described, excessive heat coupled with high rotational speeds often causes the physical connections or interfaces in the shaft and bearing assembly to loosen and vibrate. Loosening can occur when the shaft and the bearing assembly are constructed of different metals that have different thermal expansion rates. In such a case, the various parts will each expand and contract at different respective rates when heated and cooled.
By way of example, the bearing housing is typically constructed of copper, or an alloy thereof. The bearings, which are generally constructed of a steel alloy are captured in the cavity formed by the housing. As the copper housing heats up, the diameter of the cavity increases more quickly than the outside diameter of the bearings, thereby creating a gap between the bearing and the cavity wall. The gap thus defined allows the bearings to move axially within the housing so as to generate noise and vibration.
While the aforementioned problems are cause for concern in all rotating anode type x-ray tubes, they are of particular concern in the new generation of high-power rotating anode x-ray tubes which have relatively higher operating temperatures than the typical devices. In general, high-powered x-ray devices have operating powers that exceed 20 kilowatts (kw).
Attempts have been made to minimize thermal stress, strain, vibration, noise, and other effects attributable to high operating temperatures, through the use of various types of x-ray tube cooling systems. However, currently available x-ray tube cooling systems and cooling media have not been entirely satisfactory in resolving these, and other, problems in the art. By way of example, conventional x-ray tube systems often utilize some type of liquid cooling arrangement. In many of such systems, a volume of a dielectric coolant is contained in a reservoir in which the x-ray tube is disposed. An external cooling unit continuously circulates coolant through the reservoir and removes heat transmitted to the coolant by the x-ray tube.
While these types of cooling systems and cooling processes have proven adequate in some applications, they are often ineffective to manage the significant amount of heat typically produced by high-power x-ray tubes. Further, many known cooling systems are directed towards achieving an overall cooling effect with respect to the x-ray tube, but are not directed specifically to the unique cooling requirements of the bearing assembly. That is, while such systems remove heat from the x-ray tube, they may nevertheless be ineffective in removing sufficient heat from localized xe2x80x9chot spotsxe2x80x9d such as the bearing assembly. As a result, the bearing assembly may fail prematurely, thereby shortening the useful life of the x-ray device.
In light of the foregoing problems, and others, it would be an advancement in the art to provide an improved x-ray tube cooling system which provides for a relative increase in the heat removed from the x-ray tube and thereby contributes to an increase of the operational life of the x-ray tube and related components. Further, the x-ray tube cooling system should provide for an increased rate of heat transfer out of the bearing assembly and related components.
The present invention has been developed in response to the current state of the art, and in particular, in response to these and other problems and needs that have not been fully or adequately resolved by currently available x-ray tube cooling systems. Briefly summarized, embodiments of the present invention provide an x-ray tube cooling system effective in facilitating an enhanced rate of heat transfer from the bearing assembly and related components of an x-ray tube.
Embodiments of the present invention are particularly well suited for use in the context of rotating anode type x-ray tubes. However, it will be appreciated that embodiments of the present invention may be suitable for use in conjunction with various other x-ray tubes and devices where it is desired to efficiently and reliably remove heat from bearing assemblies, and related components, that are exposed to high operating temperatures.
In one embodiment of the present invention, the x-ray tube cooling system includes a heat sink joined by welding, brazing, or similar process, to the bearing housing of a rotating anode type x-ray tube. The heat sink includes a substantially solid cooling block having a plurality of extended surfaces and composed of a high heat absorption material, preferably copper or the like. In addition, the heat sink includes a shell that cooperates with the cooling block so as to define a coolant chamber substantially enclosing the extended surfaces. The shell also defines, or otherwise includes, a coolant chamber entrance and exit that are in fluid communication with the coolant chamber.
The bearing housing, wherein the bearing assembly of the x-ray tube is received, and the heat sink are supported in a cantilever position in the vacuum enclosure of the x-ray tube by a pair of insulators, preferably ceramic or the like, disposed about one end of the heat sink. Furthermore, the insulators also serve to isolate the anode from the vacuum enclosure, and thereby preserve the high voltage potential between the anode and the cathode that is required for effective operation of the x-ray device. In addition to the support function and the electrical isolation function that they provide, the insulators also cooperate with each other to define coolant inlet and outlet passageways in fluid communication with the coolant chamber entrance and exit, respectively, of the shell, and thereby permit a flow of dielectric coolant, preferably supplied by an external cooling unit, to enter the vacuum enclosure and circulate through the coolant chamber of the heat sink.
In operation, some of the heat generated during x-ray tube operations is transmitted to the bearing assembly. By virtue of the intimate and substantial contact between the bearing housing, in which the bearing assembly is received, and the substantially solid copper cooling block of the heat sink, a relatively large amount of the heat transmitted to the bearing assembly is conducted out to the cooling block. The heat present in the cooling block is continuously removed by a flow of coolant generated by the external cooling unit.
In particular, the flow of coolant is directed to the coolant chamber of the heat sink by way of the coolant inlet passageway defined by the insulators, and the coolant chamber entrance of the shell. Upon entering the coolant chamber, the flow of coolant contacts the extended surfaces of the cooling block so as to remove at least some heat therefrom. Because heat transfer rate is a function of, among other things, the total surface area over which heat is to be transferred, the extended surfaces are effective in implementing a rate of heat transfer away from the bearing assembly that is relatively greater than would otherwise be the case. After absorbing heat from the extended surfaces, the coolant then exits the coolant chamber and returns to the external cooling unit by way of the coolant chamber exit of the shell and the coolant outlet passageway defined by the insulators. The heated coolant is then cooled by the external cooling unit and directed back to the coolant chamber to repeat the cycle.
Thus, embodiments of the present invention are effective in providing, among other things, localized cooling at the bearing assembly and related components. In this way, the present invention contributes to a relative increase in the operational life of the bearing assembly and related components, and thus, the operational life of the x-ray device as a whole.
These and other aspects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.